1. Field of the Invention
Nuclear power plants are required by practice to be designed in such a manner that the health and safety of the public is assured even for the most adverse accident that can be postulated. For plants utilizing light water as a coolant the most adverse accident is considered to be a double-ended break of the largest pipe in the reactor coolant system and is termed the Loss of Coolant Accident (LOCA).
For accident protection, these plants utilize containment systems that are designed to physically contain water, steam, and any entrained fission products thay may escape from the reactor coolant system. The containment system is normally considered to encompass all structures, systems and devices that provide ultimate reliability in complete protection for any accident that may occur. Engineered safety systems are specifically designed to mitigate the consequences of an accident. Basically, the design goal of a containment system is that no radioactive material escapes from the nuclear power plant in the event of an accident so that the lives of the surrounding populace are not endangered.
The passive containment system herein disclosed provides this level of protection for a loss of coolant accident and for the other types of accidents that are considered as a basis of design, and is considered to be effective for nuclear power plants employing either Pressurized Water Reactors or Boiling Water Reactors.
2. The Prior Art
Prior art techniques have utilized either full-pressure "dry-type" containment or pressure suppression containment for light water cooled nuclear power plants.
In the full-pressure containment the reactor building, completely enclosing the reactor coolant system, is capable of withstanding the pressure and temperature rise expected from a LOCA. The building is typically constructed either of steel or steel-lined reinforced concrete or prestressed concrete.
Refinements of full-pressure containment include double leakage-control barriers and subatmospheric pressure operation. For the double leakage-control barrier any leakage into the control annulus is either pumped back into the primary containment, or the leakage is treated before being exhausted to the outside atmosphere. For subatmospheric operation the containment is normally maintained at partial vacuum, and following LOCA, the pressure is reduced back to less than the outside atmosphere utilizing active engineered safety systems to terminate any potential release of radioactivity to the environment.
The pressure-suppression containment consists of a drywell that houses the reactor coolant system, a pressure suppression chamber containing a pool of water, and a vent system connecting the drywell to the pool of water. The containment structure is constructed of steel enclosed by reinforced concrete, or is steel-lined with reinforced concrete. The pressure-suppression containment is housed within a reactor building.
In the event of a LOCA, the reactor coolant partially flashes to steam within the drywell, and the air, steam, and liquid coolant flows through the connecting vents into the pool of water in the suppression chamber. The steam is condensed by the water and decreases the potential pressure rise in the containment. The air rises into the free space above the pool of water in the suppression chamber.
Refinements in pressure-suppression containment utilizing water includes the inerting of the containment atmosphere. Inerting is aimed at preventing the burning of hydrogen evolved from metal-water reactions of overheated nuclear fuel.
A different type of pressure suppression containment utilizes an ice-condenser. The ice is maintained in a refrigerated compartment surrounding the reactor coolant system. The ice-condenser containment is divided into an upper chamber and a lower chamber with the reactor coolant system in the latter. In the event of a LOCA a pressure rise of the lower chamber causes access panels located at the bottom of the ice-storage compartment to open. This provides a flow path for air and steam through the ice bed. The steam is condensed by the ice and decreases the potential pressure rise in the containment. The air passes into the upper chamber through top access panels forced open by the flow of air.
Full-pressure containment and pressure-suppression containment are passive structures that require support systems for accident containment. Active systems such as residual heat removal systems and containment spray systems ae used to dissipate heat to the environs. This prevents the containment design pressure and temperature from being exceeded, and in the process, the containment pressure is reduced to limit the leakage of fission products. Active filtration systems are required in conjunction with the spray systems to reduce fission product concentration in the containment atmosphere. This also limits the amount of fission products that can leak out of the containment to the environs. Hydrogen recombiners are also being considered to protect the containment from developing explosive concentrations of hydrogen.
To be effective, both the full-pressure containment and the pressure-suppression containment require additional engineered safety systems that provide emergency cooling of the nuclear fuel. Some Pressurized water Reactors require passive accumulator systems in addition to active high and low pressure injection systems to maintain an adequate amount of liquid coolant at the nuclear fuel. The residual heat removal systems used for containment pressure reduction also rejects decay heat to the environs.
Pressure suppression with gravity flooding has also been proposed as an engineered safety system for the LOCA.
Active engineered safety systems are inherently required to function effectively in order to maintain the integrity of the containment system in the LOCA. Active systems require high integrity instrumentation and control equipment, rotating machinery, electric power sources and power distribution equipment. These systems need to function properly as part of a larger system under adverse containment environment conditions of high pressure, high temperature, high humidity, and high radioactivity.
Malfunctioning of any active engineered safety system imposes even more adverse conditions on the operable systems. For instance, an inadequate source of electric power may result in the malfunctioning of the emergency core cooling system for the nuclear fuel. Overheating of the fuel can result in melting of the fuel cladding with metal-water reactions occuring. The fuel core may slump and portions could collapse and overheat the bottom of the reactor vessel. Hydrogen is released from metalwater reactions and is subject to burning. The added energy from the metal-water reactions and from the burning of hydrogen imposes even more severe requirements on the containment structure. Overheating of the fuel and melting of the clad results in a gross release of fission products that are available for leakage out of the containment system. This example points to the critical nature of active engineered safety systems that are an essential part of the containment systems of the prior art.